Electrochemical capacitors (ECs), also known as ultracapacitors or supercapacitors, are being considered for uses in hybrid electric vehicles (EVs) where they can supplement a battery used in an electric car to provide bursts of power needed for rapid acceleration, the biggest technical hurdle to making battery-powered cars commercially viable. A battery would still be used for cruising, but supercapacitors (with their ability to release energy much more quickly than batteries) would kick in whenever the car needs to accelerate for merging, passing, emergency maneuvers, and hill climbing. The EC must also store sufficient energy to provide an acceptable driving range. To be cost-, volume-, and weight-effective compared to additional battery capacity they must combine adequate energy densities (volumetric and gravimetric) and power densities (volumetric and gravimetric) with long cycle life, and meet cost targets as well.
ECs are also gaining acceptance in the electronics industry as system designers become familiar with their attributes and benefits. ECs were originally developed to provide large bursts of driving energy for orbital lasers. In complementary metal oxide semiconductor (CMOS) memory backup applications, for instance, a one-Farad EC having a volume of only one-half cubic inch can replace nickel-cadmium or lithium batteries and provide backup power for months. For a given applied voltage, the stored energy in an EC associated with a given charge is half that storable in a corresponding battery system for passage of the same charge. Nevertheless, ECs are extremely attractive power sources. Compared with batteries, they require no maintenance, offer much higher cycle-life, require a very simple charging circuit, experience no “memory effect,” and are generally much safer. Physical rather than chemical energy storage is the key reason for their safe operation and extraordinarily high cycle-life. Perhaps most importantly, capacitors offer higher power density than batteries.
The high volumetric capacitance density of an EC relative to conventional capacitors (10 to 100 times greater than conventional capacitors) derives from using porous electrodes to create a large effective “plate area” and from storing energy in the diffuse double layer. This double layer, created naturally at a solid-electrolyte interface when voltage is imposed, has a thickness of only about 1 nm, thus forming an extremely small effective “plate separation.” Such a supercapacitor is commonly referred to as an electric double layer capacitor (EDLC). The double layer capacitor is based on a high surface area electrode material, such as activated carbon, immersed in a liquid electrolyte. A polarized double layer is formed at electrode-electrolyte interfaces providing high capacitance. This implies that the specific capacitance of a supercapacitor is directly proportional to the specific surface area of the electrode material. This surface area must be accessible by electrolyte and the resulting interfacial zones must be sufficiently large to accommodate the so-called electric double-layer charges.
In some ECs, stored energy is further augmented by pseudo-capacitance effects, occurring again at the solid-electrolyte interface due to electrochemical phenomena such as the redox charge transfer. Such a supercapacitor is commonly referred to as a pseudo-capacitor or redox supercapacitor. A third type of supercapacitor is a lithium-ion capacitor that contains a pre-lithiated graphite anode, an EDLC cathode (e.g. typically based on activated carbon particles), and a lithium salt electrolyte. A fourth type of supercapacitor is a sodium-ion capacitor that contains a pre-sodiated hard carbon anode, an EDLC cathode (e.g. typically based on activated carbon particles), and a sodium salt electrolyte.
However, there are several serious technical issues associated with current state-of-the-art supercapacitors:    (1) Experience with supercapacitors based on activated carbon electrodes shows that the experimentally measured capacitance is always much lower than the geometrical capacitance calculated from the measured surface area and the width of the dipole layer. For very high surface area activated carbons, typically only about 20-40 percent of the “theoretical” capacitance was observed. This disappointing performance is due to the presence of micro-pores (<2 nm, mostly <1 nm) and ascribed to inaccessibility of some pores by the electrolyte, wetting deficiencies, and/or the inability of a double layer to form successfully in pores in which the oppositely charged surfaces are less than about 1-2 nm apart. In activated carbons, depending on the source of the carbon and the heat treatment temperature, a surprising amount of surfaces can be in the form of such micro-pores that are not accessible to liquid electrolyte.    (2) Despite the high gravimetric capacitances at the electrode level (based on active material weights alone) as frequently claimed in open literature and patent documents, these electrodes unfortunately fail to provide energy storage devices with high capacities at the supercapacitor cell or pack level (based on the total cell weight or pack weight). This is due to the notion that, in these reports, the actual mass loadings of the electrodes and the apparent densities for the active materials are too low. In most cases, the active material mass loadings of the electrodes (areal density) is significantly lower than 10 mg/cm2 (areal density=the amount of active materials per electrode cross-sectional area along the electrode thickness direction) and the apparent volume density or tap density of the active material is typically less than 0.75 g/cm3 (more typically less than 0.5 g/cm3 and most typically less than 0.3 g/cm3) even for relatively large particles of activated carbon.
The low mass loading is primarily due to the inability to obtain thicker graphene-based electrodes (thicker than 100 μm) using the conventional slurry coating procedure. This is not a trivial task as one might think, and in reality the electrode thickness is not a design parameter that can be arbitrarily and freely varied for the purpose of optimizing the cell performance. Contrarily, thicker electrodes tend to become extremely brittle or of poor structural integrity and would also require the use of large amounts of binder resin. These problems are particularly acute for graphene material-based electrodes. It has not been previously possible to produce graphene-based electrodes that are thicker than 100 μm and remain highly porous with pores remaining fully accessible to liquid electrolyte. The low areal densities and low volume densities (related to thin electrodes and poor packing density) result in relatively low volumetric capacitances and low volumetric energy density of the supercapacitor cells.
With the growing demand for more compact and portable energy storage systems, there is keen interest to increase the utilization of the volume of the energy storage devices. Novel electrode materials and designs that enable high volumetric capacitances and high mass loadings are essential to achieving improved cell volumetric capacitances and energy densities.    (3) During the past decade, much work has been conducted to develop electrode materials with increased volumetric capacitances utilizing porous carbon-based materials, such as graphene, carbon nanotube-based composites, porous graphite oxide, and porous meso carbon. Although these experimental supercapacitors featuring such electrode materials can be charged and discharged at high rates and also exhibit large volumetric electrode capacitances (50 to 150 F/cm3 in most cases, based on the electrode volume), their typical active mass loading of <1 mg/cm2, tap density of <0.2 g/cm3, and electrode thicknesses of up to tens of micrometers (<<50 μm) are still significantly lower than those used in most commercially available electrochemical capacitors (i.e. 10 mg/cm2, 100-200 μm), which results in energy storage devices with relatively low areal and volumetric capacitances and low volumetric energy densities.    (4) For graphene-based supercapacitors, there are additional problems that remain to be solved, explained below:
A single graphene sheet provides a specific external surface area of approximately 2,675 m2/g (that is accessible by liquid electrolyte), as opposed to the exterior surface area of approximately 1,300 m2/g provided by a corresponding single-wall CNT (interior surface not accessible by electrolyte). The instant applicants (A. Zhamu and B. Z. Jang) and their colleagues were the first to investigate graphene-based supercapacitors [Please see Refs. 1-5 below; the 1st patent application was submitted in 2006 and issued in 2009]. After 2008, researchers began to realize the significance of graphene materials for supercapacitor applications.